Covalent Bonding
Sharing electrons, Lewis dots, geometry, and polarity.
Dr. Melinda Oliver
Naming Binary Covalent Compounds
- Nonmetal to the left on PT comes first in formula and in name. Do not change name. You will see exceptions to this "rule" especially for hydrogen.
- Use the subscripts in the formula to determine the prefixes. The figure to the right has the prefixes from 1-10. Most of these are familiar.
- If the element to the left on the PT, and, hence, in the formula, has no subscript, which means there is only one of this element in the formula, the prefix mono- is not used.
- The other nonmetal comes second in formula and in name. Drop the ending on this one and add the suffix -ide. So pretty much the same as ionic compounds.
- Do not try to figure out charges as these elements are sharing.
- Example: CO2 is carbon dioxide. Carbon is on the left and there is only one C in the formula (so no mono- prefix). N2O4 is dinotrogen tetroxide. We do not usually put two vowels next to ech other so that is why it is not tetraoxide.
Examples on naming are done along with Lewis dot structures, see below for examples.
Molecular Elements-diatomic
Phosphorus is another molecular element
Sulfur is another molecular element
Naming Acids
- Most acids are compounds that have a hydrogen atom bonded to a highly electronegative atom so that the hydrogen can be "taken" by the action of hydrogen bonding with another molecule like water. When this happens, hydrogen has no electrons and we call it a "proton" because it is essentially just a proton (because the most common isotope of hydrogen has no neutrons).
- Binary acids (H and another element) are named use the hydro- prefix and the -ic ending for the other element. For example, HBr is hydrobromic acid and H2S is hydrosulfuric acid.
- Acids that have hydrogen and oxygen are the so-called oxyacids and their names come from the polyatomic ions that combine with hydrogen to make the acid. If the polyatomic ion ends with -ate like sulfate, nitrate, etc become -ic in the acid form.
- Polyatomic ions ending in -ite like nitrite become -ous in the acid form. So nitrous acid has the formula, HNO2 and H2SO3 is named sulfurous acid.
Lewis Dots
Follow these simple steps to draw Lewis dot structures: (do not micromanage electrons!)
- Add up valence electrons for all atoms in the molecule. Add for negative/subtract for positive charges (polyatomic ions).
- Connect atoms with bonding “dashes” (single bonds) indicating sharing of electrons.
- Give each atom 8 electrons EXCEPT FOR B and H. (Li and Be are exceptions too but not seen often as they are METALS).
- Count electrons.
- If there are too many, look for C, N, O, P, and S atoms. If these atoms are present they can share more than 2 electrons in a multiple bond WITH EACH OTHER. Sharing 4 is a double bond. Sharing 6 is a triple bond.
- If there are too few, the central atom can share more than 8 IF THAT ATOM IS 3RD ROW OR BELOW ON THE P.T.
Molecular Geometry (VSEPR)
This link has loads of background info on all of the shapes:
http://intro.chem.okstate.edu/1314F97/Chapter9/VSEPR.html
You must draw the Lewis dot structure first. FOCUS on the central atom and count number of dashes and pairs of dots. Double and triple bonds count as one set of dashes. This will be the steric number. Find that row on the geometry sheet (in notes and in resources). Then find which shape corresponds to the number of dashes (bonds) and the number of pairs of dots, lone pairs.
Resonance
Polarity
- Check polarity of bonds in the molecule using the table of electronegativities.
- If bonds are nonpolar, STOP, the molecule is non polar.
- If bonds are polar, check geometry sheet to see if the geometry cancels out the polarity*.
These steps are explained starting at 16:30 in lecture 10.
Examples at the top on pg 28 in the notes begin at 32:28 in second video linked above.
Geometry Chart and Polarity
A “No” on the geometry sheet means that the geometry DOES cancel the polarity of the bonds and the molecule is, therefore, nonpolar. A “Yes” means that the polarity is NOT CANCELED and the molecule is, therefore, polar.
*IMPORTANT: The “Yes” and “No” on the geometry sheet are for molecules that have all bonds the same around the central atom like CF4 and CO2 but if the molecule is CH3F, all bonds are not the same and the polarities of the C-F and the C-H bonds are different so they do not cancel.
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Have Questions Later?
Dr. Melinda Oliver
University of South Alabama
Email: oliver@southalabama.edu
Location: University of South Alabama, Mobile, AL, United States
Phone: (251)405-4504